Ah, for a given anode current yes I suppose that is true. Hadn't thought of that. I was still thinking in terms of a given bias voltage.
> Langford Smith "Radiotron", chapter 12.
> Another book.
> More from G. Valley, H. Wallman
Yes, the ancients have stolen ALL our proofs.
> thinking in terms of a given bias voltage.
Everything is on the table. All resistors can be changed.
The core of the point is that maximum G1 swing will give maximum plate swing. And the maximum G1 swing is liable to be very near Vg2/Mu(g2). Therefore for maximum -gain- you want low G2. This extends to signals less than max-- low G2 is higher gain.
This may starve the tube so bad you can't drive the load. However most times you want "max gain!" the signal is so small that even the output is not-large. (Guitar amps an exception because guitar levels may be crazy.)
"Driving the load" obviously includes stray capacitance. A 5Meg Rp gives high gain for low freqs but will suck by 1KHz. Wallman et al do show similar hi-Z loading for low-freq amps for gun aiming (servo speed limited to sub-CPS by motoring a 2-ton cannon around).
In the OP's plan, assuming 6SN7 cathode at 8V, 6SJ7 Mu near 20, the max input is 8V/20 or 0.4V peak. This is not out of line for 1948 pickups and playing styles (thump behind the horns). Today's hot players (humbuckers, boosters, titanium picks) will find a break-up which can't be avoided with the amp's gain knob. Kewl!
> My concern was that the screen current draw, not being isolated from the source of voltage at the cathode of the 6SN7 by (an expected) larger resistor and bypass cap, would cause a drop over the cathode resistor, and swing
Draw little arrows. Ignore the 20uFd. It is NFB. Gain may be a hair down. As Mu(g2) is near 20, gain might be 5% off. If the 20uFd is healthy, it is nothing.
> PRR has it backwards?! Surely you jest...
Me and my fingers can do backward also. Glad someone is checking.
> Another book.
> More from G. Valley, H. Wallman
Yes, the ancients have stolen ALL our proofs.
> thinking in terms of a given bias voltage.
Everything is on the table. All resistors can be changed.
The core of the point is that maximum G1 swing will give maximum plate swing. And the maximum G1 swing is liable to be very near Vg2/Mu(g2). Therefore for maximum -gain- you want low G2. This extends to signals less than max-- low G2 is higher gain.
This may starve the tube so bad you can't drive the load. However most times you want "max gain!" the signal is so small that even the output is not-large. (Guitar amps an exception because guitar levels may be crazy.)
"Driving the load" obviously includes stray capacitance. A 5Meg Rp gives high gain for low freqs but will suck by 1KHz. Wallman et al do show similar hi-Z loading for low-freq amps for gun aiming (servo speed limited to sub-CPS by motoring a 2-ton cannon around).
In the OP's plan, assuming 6SN7 cathode at 8V, 6SJ7 Mu near 20, the max input is 8V/20 or 0.4V peak. This is not out of line for 1948 pickups and playing styles (thump behind the horns). Today's hot players (humbuckers, boosters, titanium picks) will find a break-up which can't be avoided with the amp's gain knob. Kewl!
> My concern was that the screen current draw, not being isolated from the source of voltage at the cathode of the 6SN7 by (an expected) larger resistor and bypass cap, would cause a drop over the cathode resistor, and swing
Draw little arrows. Ignore the 20uFd. It is NFB. Gain may be a hair down. As Mu(g2) is near 20, gain might be 5% off. If the 20uFd is healthy, it is nothing.
> PRR has it backwards?! Surely you jest...
Me and my fingers can do backward also. Glad someone is checking.
I guess my failure was looking at the resistor values and trying to estimate the bias point of both the 6SJ7 and 6SN7.
The subsequent 6SN7 is run below textbook values. With a 4700R cathode resistor and two tubes in parallel at -8Vgk, it would see a bias current less than 1mA.
The circuit seems "just all wrong" if one looks at recommended RCA (Resistance Coupled Amplifier) values.
This was an instrument (Dobro) amplifier from what I read. As such it was not designed for maximum linearity.
The subsequent 6SN7 is run below textbook values. With a 4700R cathode resistor and two tubes in parallel at -8Vgk, it would see a bias current less than 1mA.
The circuit seems "just all wrong" if one looks at recommended RCA (Resistance Coupled Amplifier) values.
This was an instrument (Dobro) amplifier from what I read. As such it was not designed for maximum linearity.
I had read most of this old books, and fortunately the two Terman's are waiting in my brother's house to be read too. I love this old text books, and I have a relatively good photographic memory, so I can find relatively easy the place I read and then search in the PDF copy and paste here. I always prefer the printed book over the pdf, but it is easy to paste and give the true answer.
Re-examining the first 6SN7 stage, I get about 3.5V at cathode.
This does seem mighty low.
None of these old Gibson plans can be taken too seriously. The mandolins and guitars were good, the amps were tossed-together, and sometimes the guts had little to do with the model badge or any schematic.
This does seem mighty low.
None of these old Gibson plans can be taken too seriously. The mandolins and guitars were good, the amps were tossed-together, and sometimes the guts had little to do with the model badge or any schematic.
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